Power limiting apparatus



W. T. WINTRINGHAM POWER LIMITING APPARATUS May 26, 1931.

Filed May 14, 1929 R R KE YINVENTOR WZ BY ATTORN EY Patented May 26, 1931 TEE UNITED ST WILLIAM '1. WINTRINGHAM, 0F BROOKLYN, NEW YORK, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK POWER LIMITING APPARATUS Application filed May 14, 1929. Serial No. 363,072.

This invention relates to Communication systems, and more particularly to a method of and means for limiting power or voltages applied to apparatus included in such systems. V

A problem, which is of general occurrence in the communication art, particularly where high powered amplifiers are concerned, as in a radio transmitting station, is that of preventing overload of vacuum tube or other devices in the circuit, which causes distor c tion of the signals andmay cause breakdown of tubes due to excessive voltages. This invention presents a novel method of and means for the solution of problems of the above general type.

In a transmission system utilizing the power limiting'method and arrangements of this invention, the signal transmission path will be divided into two branches. Through one of these branches, provided with an amplifier and a delay circuit, the signals pass in the ordinary way. The other path is so arranged by means of vacuum tube circuits that for values of signals below the desired cut off point, little or no power is transmitted through it, thereby leaving the firstipath to be normally the dominant one. The vacuum tube arrange ment of the second branch is such that when the signalling power approaches the point at which cut-off is desired, the transmission effectiveness of the second branch increases. By placing the output circuits of the two branch paths in opposition, this transmission at the higher levels through the second branch is made to oppose the normal transmission through the first branch, thereby effecting a limiting value of the overall transmission through the circuit. 7

In a number of the voltage limiting methods of the prior art, the main transmission path is through the limiting element of the circuit. One of the outstanding differences in principle between methods and arrangements of the prior art and the method and arrangements'of thisinvention lies in the fact that the main transmission path in the arrangements of this invention is distinct from the variable circuit elements.

'This fundamental difference between the voltagelimiting methods of the prior art and those herein describedmakes it possible in the utilization of this method to greatly reduce the tendency for the voltage limiting arrangements to introduce distortion in the signals. Other features and objects of the invention will appear more fully from the detailed description thereof hereinafter given. I

The invention may bermore fully understood from the following detailed description thereof,together with the accompanying drawing in the figure of which is shown a circuit diagram embodying apreferred form of the invention. H v

In the. drawing'is shown a section 1 of a transmission line. This line section is divided by transformer 2' into two branch sections 3 and 8. Line section 3 is the main transmission path and includes an amplifier 4 and a delay circuit D It is connected through transformer 5 to an;out-put linesection 6 which may include filter apparatus 7. The line section 8 includes a variable "attenuator 9, a delay circuit D a variable phase shifter PS a transformer 10 and an amplifier comprising the vacuum tubes VT and VT -The' output of this amplifier is connected through transformer 5 to the output circuit 6. It is pointed out that at a point such as 4:0 in theoutput of the amplitier the conductors will be turned over. This is to secure phase opposition in transformer 5 between the currents in circuits 3, and 8. The gain through this branch'path 8 is variable, its value being low and substantiallyconstant for all input signals less than the predetermined safe value, and increasing as the input signal increases above this safe value.- By suitable adjustments of the variable phase shift device PS the sig- VT and VT is varied by changing the grid bias on these tubes. The grid bias is supplied in part from the detector-amplifier system comprising detector tube VT and amplifier tube VT This detectoramplifier is connected by the tube VT and the circuit 13 in parallel with the input circuit 8 of tube VT. and VT If we write the plate current of VT where L is the plate current of this tube under the condition of no input signal, and Ai the increase in current due to the rectification of the signal, the voltage at the grid VT is e [E +I R A'Z R and at the grid of VT 5 Accordingly, it is evident that the bias on the tubes VT and VT is large with no signal at the input of this device and that it decreases as the signal is increased. It is possible then to so apportion the battery potentials and the resistances in these C11- cuits so that the gain of the amplifier comprising in part tubes VT and VT is small for signal inputs below a definite value and increases for increase in input thereafter. By this method and means the power or voltage applied to the output line section 6 may be limited to a desired value.

The delay circuit D is included between the rectifier input and the amplifier input circuits so that a large peak of voltage may change the gain of the amplifier comprising tubes VT and VT before it is transmitted through this amplifier. The delay circuit D retards this same voltage peak by the amount of the total delay in the other branch path so that it reaches the transformer 5 at the same instant through both paths. The variable phase shifter PS allows small variations in the transmission time so that the phase difference between the two paths actually is 180 degrees so that the signals through the two paths substract. The amplifier 4- is included in the path 3 to prevent singing in the circuit.

Using the specific method of this invention for changing the gain of the variable gain amplifier VT VT the distortion produced in this amplifier is large when its gain is small and small when its gain is large. When the distortion of this amplifier is large, however, the output from it is small compared to the signal in the other transmission path. Accordingly, the overall distortion produced by these arrangements is comparatively small. The band-pass filter 7 in the output circuit serves to remove all these components of the resultant distortion which do not fall back into the transmitted band.

Since the limiting action in the voltage limiting device of this invention is produced by a difference method, it is evident that the distortion in the output signal is materially less when the limiting is accomplished by the method of this invention rather than with former methods heretofore referred to. To further illustrate this fact, suppose thatthe input signal is 10 per cent. greater than the safe value (say 1.10 E where E, is the safe voltage). The limiting circuit must supply a bucking voltage equal to .lOE to reduce the output to the safe value. Even if the limiting branch of the circuit distorts the applied signal very badly, so much so that its output contains 50 per cent. distortion components, the actual distortion in the output is only .1015 X .5 or 5 per cent. As the limiting branch is called upon to supply an increasing bucking voltage, the distortion in it as a circuit element decreases, so that the overall distortion over the entire limiting range is low. The use of the method of the invention greatly decreases the residual distortion by virtue of the differential principle no matter what is the means used to reduce distortion in the nonlinear amplifying portion of the voltage limiter.

lVhile the arrangements of the invention have been referred to as applicable to a high frequency radio system, it is pointed out that they are applicable to other types of transmission systems. For example, they might be applied to volume control in voice frequency circuits, provided a proper network were provided in the rectifier circuit so that the change in gain would be delayed by an amount greater than the syllable period preventing operation on any except permanent changes in volume. Accordingly, while the invention has been disclosed as embodied in certain specific arrangements which are deemed desirable, it is understood that it is capable of embodiment in many other forms without departing from the spirit of the invention as defined by the appended claims.

What is claimed is:

1. A voltage limiting apparatus comprising two branch circuits having their outputs connected in opposing relation to an outgoing circuit in which the voltage is to. be limited, one of said branch circuits transmitting waves of all amplitudes applied to the system without substantial distortion, and

means to substantially prevent transmission over the other branch until the applied potentials bear a predetermined relation to the potential to which the outgoing circuit is to be limited, whereby when the applied potentials are below the predetermined potential transmission to the outgoing circuit takes place exclusively from said first branch and when the applied potential exceeds said predetermined potential the resultant waves transmitted over the second branch substantially compensate for the wave excess transmitted over said first branch.

2. A voltage limiting apparatus comprising two branch circuits having their outputs connected in opposing relation to an outgoing circuit in which the voltage is to be limited, one of said branch circuits transmitting waves of all amplitudes applied to the system without substantial distortion, means to substantially prevent transmission over the other branch until the applied potentials bear a predetermined relation to the potential to which the outgoing circuit is to be limited, and means to cause the waves in said second branch to be opposite in phase to the waves in said first branch, whereby when the applied potentials are below the predetermined potential transmission to the outgoing circuit takes place exclusively from said first branch and when the applied potential exceeds said predetermined potential the resultant waves transmitted over the second branch substantially compensate for the wave excess transmitted over said first branch.

3. A voltage limiting apparatus compris ing two branch circuits having their outputs connected in opposing relation to an outgoing circuit in which the voltage is to be limited, one of said branch circuits in cluding an amplifier having uniform'gain for waves of all amplitudes'applied to the system, and an amplifier in the other branch circuit having means controlled by the voltage applied thereto so that transmission over the second branch is substantially prevented until the potentials applied to the second amplifier bear a predetermined relation to the potential to which the outgoing circuit is to be limited, whereby when the applied potentials are below the predetermined potentialtransmission to the outgoing circuit takes place exclusively from said first branch and when the applied potential exceeds said predetermined potential the resultant waves transmitted over the second branch substantially compensate for the wave excess transmitted over said first branch. 7

4. A voltage limiting apparatus compris ing two branch circuits having their outputs connected in opposing relation to an outgoing circuit in Which'the voltage is to be limited, one ofsaid branch circuits includ ingan amplifier having thesame gain for waves for all amplitudes applied to the system, a vacuum tube amplifier in the second branch, and means for changing the grid bias of said second amplifier in accordance with the input voltage of said second amplifier to substantially prevent transmission over said branch'until the applied potentials bear a'predetermined relation to the potential to which the outgoing circuit is to be limited, whereby when the applied potentials are below the predetermined potential transmission to the outgoing circuit takes place exclusively from said first branch and when the applied potential ex ceeds said predetermined potential the resultant waves transmitted over the second branch substantially compensate for the wave excess transmitted over said first branch. V

5. A voltage limiting apparatus comprising two branch circuits having their outputs connected in opposing relation to 'anoutgoing circuit in which the voltage is to be limited, one of said branches'including an amplifier having substantially the same gain forwaves of all amplitudesapplied to the system, an amplifier in the second branch, a rectifier circuit in parallel with said second branclnand means for connecting the rectifier output to the amplifier in saidsecond branch to so control the gain thereof as to substantially prevent transmission over said second branchuntil theapplied potentials bear a predetermined relation to the potential to which the outgoing circuit is to be limited, whereby when the applied potentials arebelow the predetermined potential transmission to the outgoing circuit takes place exclusively from said first branch and when the applied potential exceeds said predetermined potential the resultant waves transmitted over the second branch substantially compensate for the wave excess transmitted over said first branch.

6. A voltage limiting apparatus comprising two branch circuits having their outputs connected in opposing relation to an outgoing circuit in which the voltage is to be limited, one of said branches including an amplifier having the same gain for Waves of all amplitudes applied to the system, an amplifier in said second branch having its grid normally so biased as to substantially prevent transmission until the applied potentials bear a predetermined relation to the potential to which the outgoing circuit is to be limited, a rectifier circuit in parallel to the second branch, said rectifier circuit being so associated with the amplifier in said second branch as to change the grid bias of said amplifier in response to the applied waves to render said amplifier'eli'ective to produce in its output waves whoseamplitude is equivalent to the excess amplitude of'the input waves applied to the system, whereby when the applied potentials are below the predetermined potential transmission to the outgoing circuit takes place exclusively from said first branch and when the applied potential exceeds said predetermined potential the resultant waves transmitted over the second branch substantially compensate for the wave excess transmitted over said first branch.

7. A voltage limiting apparatus comprising two branch circuits having their outputs connected in opposing relation to an outgoing circuit in which the voltage is to be limited, one of said branch circuits including an amplifier having the same gain for waves of all amplitude applied to the system, an amplifier in the second branch, and means associated with said second amplifier and responsive to applied waves to render the gain of said second amplifier high when the voltage applied thereto is high, and low when the voltage applied thereto is low.

8. A voltage limiting apparatus comprising two branch circuits having their outputs connected in opposing relation to an outgoing circuit in which the voltage is to be limited, one of said branch circuits including an amplifier having the same gain for waves of all amplitude applied to the system, an amplifier in the second branch, and means associated with the amplifier in said second branch and responsive to applied waves to control the gain of said amplifier so that the gain of said amplifier will be high when the voltage applied thereto is substantially above a limiting voltage, and low when the voltage applied thereto is substantially below a limiting voltage.

In testimony whereof, I have signed my name to this specification this 13th day of May, 1929.

WILLIAM T. WINTRINGHAM. 

